Wind-induced oscillation

Steel chimneys are light, wind-sensitive structures. Not only do steel chimneys move with the wind, but the circular cross section also induces the shedding of wind vortices causing movement at right angles to the wind.

This movement can become excessive when the alternating wind force coincides with the natural frequency of the chimney. The stresses generated in the chimney may be much greater than the normal wind load stresses for which the chimney is designed.

Excessive wind-induced oscillation used to be controlled by helical strakes fixed to the top section of the chimney but nowadays a damped pendulum or other positive damping system is used.

Connecting joints

Steel chimneys are fabricated from a series of individual sections that are short enough for transport. The sections are connected together during chimney erection.

The joints comprise steel flanges that are bolted together. If designed properly, the joints require only routine maintenance. Unfortunately some chimneys have inadequately designed jointing flanges.

The most common consequence is early failure of the bolts. If the early warning signs are neglected, failure of the chimney can result.

Corrosion

The UK Health and Safety Executive is so concerned about the dangers of neglected steel chimneys, and particularly the effects of hidden corrosion, that it has produced its own guidance note for chimney owners.

Corrosion is most frequently caused by a combination of sulphur-containing fuels and flue gas temperatures that are too low. The resulting condensates of sulphur acids attack the steel wall of the flue. Where the flue also forms the structural chimney windshield, the corrosion weakens the chimney.

Steel chimneys weakened by corrosion can fail with catastrophic results.

Ancillary fittings

As well as fulfilling the purpose of discharging flue gases at a height to suit environmental requirements and having the structural capacity to withstand the design wind loading, chimneys often have ancillary fittings such as lightning protection, and aircraft warning lights.

Multi-flue chimneys may have access ladders and platforms and internal lighting. All these fittings require maintenance to be safe.